Radio frequency identification system and a method of operating same

ABSTRACT

A tag receives wireless signpost signals that include timing data, determines time information associated with an event as a function of the timing data, and transmits a wireless tag signal that contains the time information. In a another configuration, a signpost transmits wireless signpost signals having a selected transmission range; a reader proximate to the signpost receives wireless tag signals with a reception range that is approximately the same as the selected transmission range; and a tag movable relative to the signpost and reader responds to receipt of a wireless signpost signal by transmitting a wireless tag signal containing a tag code associated with the tag.

FIELD OF THE INVENTION

This invention relates in general to tracking techniques and, moreparticularly, to techniques for tracking items or vehicles using radiofrequency identification technology.

BACKGROUND

According to an existing technique for tracking items or vehicles, adevice known as a radio frequency identification (RFID) tag is mountedon each item or vehicle. Signposts that transmit short-range signpostsignals are provided near locations where tags will likely pass, forexample near a door through which tags routinely travel. The tags canreceive the signpost signals from nearby signposts, and can alsotransmit wireless tag signals that include information from thesignpost. The tag signals typically have a an effective transmissionrange that is significantly longer than the effective transmission rangeof the signpost signals. Stationary devices commonly known as readersare provided to receive the tag signals. Existing systems of this typehave been generally adequate for their intended purposes, but have notbeen satisfactory in all respects.

SUMMARY OF THE INVENTION

One of the broader forms of the invention involves: receiving in areceiver section of a tag wireless signpost signals that include timingdata; responding to the occurrence of an event by determining in afurther section of the tag time information associated with the event asa function of the timing data from a received wireless signal; andtransmitting from a transmitter section of the tag wireless tag signalsthat each include a tag code associated with the tag, includingresponding to the determining of the time information by the furthersection by causing the transmitter section to include the timeinformation in at least one tag signal.

Another of the broader forms of the invention involves: transmittingfrom a signpost wireless signpost signals having a selected transmissionrange; receiving in a reader located proximate the signpost wireless tagsignals with a reception range that is approximately the same as theselected transmission range; moving a tag relative to the signpost andthe reader; receiving in a receiver section of the tag the wirelesssignpost signals; and responding to receipt by the receiver section of awireless signpost signal by transmitting from a transmitter section ofthe tag a wireless tag signal that includes a tag code associated withthe tag.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be realized fromthe detailed description that follows, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram showing an apparatus that embodies aspects ofthe present invention, and that includes a signpost, a beacon tag, areader and a central control system.

FIG. 2 is a diagrammatic view of a digital word that representsinformation transmitted by the signpost of FIG. 1 within a wirelesssignpost signal.

FIG. 3 is a diagrammatic view of a digital word that representsinformation transmitted by the tag of FIG. 1 within a wireless tagsignal.

FIG. 4 is a diagrammatic top view showing one possible application forthe apparatus of FIG. 1.

FIG. 5 shows an example of a location table that is stored within thetag of FIG. 1.

FIG. 6 shows an example of a replacement/active table that is storedwithin the tag of FIG. 1.

FIG. 7 shows an example of an equivalent table that is stored within thetag of FIG. 1.

FIG. 8 shows an example of a sequence table that is stored within thetag of FIG. 1.

FIG. 9 is a flowchart showing how the tag of FIG. 1 utilizes the tablesof FIGS. 5-8 when the tag receives a signpost signal.

FIG. 10 is a diagrammatic fragmentary view of a hypothetical scenariorepresenting another possible application for the apparatus of FIG. 1.

FIG. 11 is a diagrammatic view of a scenario that represents anapplication for an apparatus that is an alternative embodiment of theapparatus shown in FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a block diagram showing an apparatus 10 that embodies aspectsof the present invention. The apparatus 10 includes a signpost 11, abeacon tag 12, a reader 13 and a central system 14. The apparatus 10actually includes a number of signposts of the type shown at 11, anumber of tags of the type shown at 12, and several readers of the typeshown at 13. However, for clarity in explaining aspects of the presentinvention, FIG. 1 shows only one signpost 11, one tag 12 and one reader13.

The signpost 11, reader 13 and central system 14 have respective networkinterfaces 16, 17, and 18 that are operatively coupled to a network 19.In the disclosed embodiment, the network 19 conforms to an industrystandard commonly known as an Ethernet network. However, the network 19could alternatively be any other suitable type of network, and couldinclude wireless links. In the disclosed embodiment, the signpost 11,reader 13 and central system 14 are stationary, whereas the tag 12 ismobile. For example, the tag 12 may be supported on a vehicle, or on anitem such as a shipping container. However, the invention encompassesalternative configurations in which the tag 12 is stationary, and one ormore of the other components are mobile.

The signpost 11 includes a control circuit 26 that is operativelycoupled to the network interface 16. The control circuit 26 may be atype of circuit commonly known as a microcontroller. The control circuit26 includes a processor 27 and a memory 28. The memory 28 stores anidentification code 31. In the embodiment of FIG. 1, each signpost 11has a different identification code 31, such that each identificationcode 31 uniquely identifies a particular signpost. The identificationcode 31 does not change during normal operation of the system shown inFIG. 1. The memory 28 also stores suppression on/off information 32, andantenna select information 33, for purposes that are discussed in moredetail later.

The signpost 11 includes a real-time clock (RTC) circuit 36 that isoperatively coupled to the control circuit 26. The signpost 11 alsoincludes a low frequency (LF) antenna 37, and an LF transmitter circuit38 that is operatively coupled to the control circuit 26 and the antenna37. The control circuit 26 can transmit LF wireless signpost signals 39through the transmitter 38 and antenna 37. The transmitter 38 is a typeof circuit known in the art, and is therefore not illustrated anddescribed here in detail. The antenna 37 is a ferrite core and/or planarcoil antenna of a known type. The antenna 37 is configured to transmitan omni-directional signal, but the antenna could alternatively beconfigured to transmit a signal that is to some extent directional.

The transmitter 38 generates the signpost signal 39 by effectingamplitude modulation of a carrier signal having a frequency within arange of approximately 30 KHz to 30 MHz. In the embodiment of FIG. 1, inorder to facilitate compliance with governmental regulations of variousdifferent countries regarding electromagnetic emissions, the carrierfrequency is selected to be 132 KHz. However, the carrier frequencycould alternatively be some other frequency, such as 125 KHz or 13.56MHz.

The transmitter 38 and the antenna 37 are configured so that thewireless signpost signals 39 are near-field signals of primarilymagnetic character. As known to persons skilled in the art, a wirelesssignal with near-field characteristics has a roll-off that is roughlythree times higher than the roll-off for a signal with far fieldcharacteristics. Consequently, the signpost signals 39 intentionallyhave a relatively short transmission range. This short transmissionrange can be adjusted to some extent. In the embodiment of FIG. 1, thetransmission range is selected to be about 4 to 12 feet. Since thesignpost signals 39 have near field characteristics, the transmissionand reception of the signpost signals 39 may be viewed as fundamentallya magnetic coupling between two antennas, rather then a radio frequency(RF) coupling. The localized nature of the signpost signals 39 havingnear-field characteristics helps to facilitate compliance withgovernmental regulations, and also helps to minimize reception of thesewireless signals by tags 12 that are beyond an intended transmissionrange of the signpost signals 39.

The wireless signpost signal 39 is typically transmitted in a relativelynoisy environment. In order to ensure reliable signal detection by tags(such as the tag 12), known techniques are used to improve thesignal-to-noise ratio (SNR). For example, in order to improve the SNR inthe embodiment of FIG. 1, the amplitude modulation of the 132 KHzcarrier is effected using the well-known technique of amplitude shiftkeying (ASK). It would alternatively be possible to use either frequencyshift keying (FSK) or phase shift keying (PSK), in order to achieve aneven higher SNR. However, use of FSK or PSK would typically requireadditional analog circuitry within each tag 12. Therefore, and since oneobject of the invention is to implement both the signpost 11 and the tag12 at a low cost, ASK is used in the embodiment of FIG. 1.

Turning to the tag 12, two LF antennas 43 and 44 are orientedorthogonally with respect to each other. The tag 12 also includes an RFantenna 46. The RF antenna 46 is omni-directional, but it couldalternatively be configured to be directional. The tag 12 has a controlcircuit 47 that includes a processor 48, an LF receiver 49 coupled tothe LF antennas 43 and 44, an RF transmitter 51 coupled to the RFantenna 46, and an RF receiver 52 coupled to the RF antenna 46. The LFreceiver 49 receives the wireless signpost signals 39 using one or bothof the LF antennas 43 and 44. The receiver 49 is capable of detectingwhether or not one or both of the antennas 43 and 44 are currentlywithin the magnetic field generated by the antenna 37 of any signpost11.

The reader 13 can transmit ultra high frequency (UHF) wireless signals54, and the control circuit 47 of the tag 12 can receive these wirelesssignals 54 through the RF antenna 46 and the RF receiver 52. The controlcircuit 47 can transmit UHF wireless beacon or tag signals 53 using theRF transmitter 51 and the RF antenna 46. In the embodiment of FIG. 1,the wireless tag signals 53 are generated by using FSK modulation tosuperimpose selected information onto a carrier signal. The carriersignal has a frequency of 433.92 MHz, but it could alternatively havesome other suitable frequency. One suitable alternative frequency is 915MHz. Under current governmental regulations for transmission ofelectromagnetic signals, the frequency of 433.92 MHz is available foruse in a larger number of countries then the frequency of 915 MHz.Consequently, the embodiment of FIG. 1 uses the frequency of 433.92 MHz.

The wireless tag signals 53 are transmitted using a technique that isknown in the art as a slotted aloha protocol, in order to reduceinterference between tag signals transmitted by the tag 12, similar tagsignals transmitted by other tags, and the wireless signals 54transmitted by the reader 13. The effective transmission range of thewireless signals 53 and 54 is significantly longer than the effectivetransmission range of the wireless signpost signals 39. In theembodiment of FIG. 1, the wireless signals 53 and 54 each have aneffective transmission range of approximately 300 feet. In contrast, asmentioned above, the wireless signpost signals have an effectivetransmission range of about 4 to 12 feet.

The tag 12 has a memory 59. The memory 59 is operatively coupled to thecontrol circuit 47, and stores a not-illustrated program that isexecuted by the processor 48. The memory 59 also stores four tables61-64, for a purpose discussed in more detail later. In the embodimentof FIG. 1, the information in the tables 61-64 is periodically updatedby the central system 14. In particular, the central system 14 providesupdate information for the tables to the reader 13, and the reader 13then transmits this update information within wireless signals 54. Whenthe tag 12 receives the wireless signals 54 containing the updateinformation, the tag 12 updates the tables 61-64. Alternatively, thetables 61-64 could be updated by temporarily inserting the tag 12 into anot-illustrated docking station of a known type that is coupled to thecentral system 14, and that allows the central system 14 to communicatewith the control circuit 47 in the tag 12.

The tag 12 includes an RTC circuit 57 that is operatively coupled to thecontrol circuit 47, and includes a sensor 58 that is also operativelycoupled to the control circuit 47. In FIG. 1, the sensor 58 measurestemperature. However, it could alternatively measure or detect someother parameter, such as humidity, the integrity of a seal securing thedoor of a shipping container, or some other parameter.

The reader 13 is a device of a type generally known in the art.Therefore, the internal structure of the reader 13 is not shown anddescribed here in detail, and the following discussion addressesprimarily the unique characteristics of the reader 13 that relate toaspects of the invention. The reader 13 can transmit wireless signals at54, and the control circuit 47 of tag 12 can receive the wirelesssignals 54 through the antenna 46 and the receiver 52. In the embodimentof FIG. 1, the wireless signals 54 are UHF signals that have a frequencyof 433.92 MHz, with Manchester encoded FSK modulation at 27.7 KBPS.

The central system 14 is an arrangement of a type generally known in theart. Therefore, the internal structure of the central system 14 is notshown and described here in detail. Instead, the following discussionaddresses primarily the unique characteristics of the central system 14that relate to aspects of the invention. In addition to the networkinterface 18 that was mentioned above, the central system 14 has an RTCcircuit 71 that accurately keeps track of time.

FIG. 2 is a diagrammatic view of a digital word 101 that representsinformation transmitted by the signpost 11 of FIG. 1 within its signpostsignal 39. In more detail, with reference to FIGS. 1 and 2, the bits ofthe digital word 101 are incorporated into the signpost signal 39 byusing amplitude modulation to serially modulate the bits of the word 101onto the 132 KHz carrier. The bits of the word 101 are transmittedserially from left to right in FIG. 2. The digital word 101 includesseveral fields 106-112.

The first field is a preamble 106, and is a predefined pattern of bitsthat will allow a device receiving the wireless signpost signal 39 torecognize that the signpost signal is beginning, and to then synchronizeitself to the signpost signal. The next field 107 in the word 101 is asignpost code, and in particular is the identification code 31 from thememory 28 of the signpost 11. As mentioned earlier, the system of FIG. 1has a number of different signposts 11, only one of which is shown inFIG. 1. Since each signpost uses a different signpost code 107, thesystem can distinguish signpost signals transmitted by one signpost fromsignpost signals transmitted by other signposts.

The next field 108 in the digital word 101 contains timing information.In this regard, as explained above, the central system has an RTC 71that maintains accurate time information. The central system 14periodically sends timing information from its RTC 71 through thenetwork 19 to the signpost 11, and the signpost 11 uses this timinginformation to update its own RTC 36, so that the RTC 36 is synchronizedto the RTC 71 and thus is also very accurate. When the signpost 11transmits its wireless signpost signal 39, it takes current timinginformation from its own RTC 36, and puts this timing information intothe field 108 in the digital word 101. When the tag 12 receives thewireless signpost signal 39, it uses the timing information at 108 toupdate its own RTC 57. Thus, when the tag 12 is in the region of thesignpost 11, the RTC 57 in the tag 12 will be closely synchronized withthe RTC 36 in the signpost 11 and also with the RTC 71 in the centralsystem 14, and thus will be very accurate.

As an alternative approach, timing information from the RTC 71 could intheory be supplied from the central system 14 to the reader 13, andcould then be sent to the tag 12 within the wireless signals 54.However, communication between the tag 12 and reader 13 in the form ofwireless signals 53 and 54 involves timing considerations. For example,after sending a wireless signal 54, the reader 13 may have to wait for aperiod of time before sending another wireless signal 54, in order toprovide a time interval during which a number of tags 12 can transmitwireless signals 53 according to the slotted aloha protocol mentionedabove. Suppressing transmission of the signals 54 during this timeinterval avoids having the signals 54 interfere with signals 53transmitted by the tags. Consequently, the transmission of wirelesssignals 54 by the reader 13 can be sporadic, and it becomes problematicto achieve accurate and reliable delivery of timing information to thetags 12 through the RF wireless signals 54. In contrast, each signpost11 can transmit its wireless signpost signals 53 on a relatively regularbasis, and thus it is possible to achieve accurate and reliable deliveryof timing information to the tags 12 using the LF wireless signpostsignals 39.

The next field in the digital word 101 is an antenna select field 109.The signpost 11 inserts in this field the antenna select informationstored at 33 in its memory 28. In the embodiment of FIG. 1, the antennaselect information 33 indicates that antenna select capability is eitherenabled or disabled. Thus, for example, the antenna select informationcould be a single binary bit that is a binary “1” when the antennaselect capability is enabled, and a binary “0” when the antenna selectcapability is disabled.

When the tag 12 receives the signpost signal 39, the tag 12 looks at theantenna select field 109 to see if the antenna select capability isenabled or disabled. If the field 109 indicates that the antenna selectcapability is enabled, then the tag 12 uses the LF receiver 49 todetermine which of the LF antennas 43 and 44 is currently producing astronger signal in response to the magnetic field generated by a nearbysignpost 11. The control circuit 47 then disables the other of theantennas 43 and 44, or in other words the antenna that is producing theweaker signal. The tag 12 then continues operating with only one of theantennas 43 and 44, until it receives a further wireless signpost signal39 in which the field 109 indicates that the antenna select capabilityis to be disabled. Upon receiving a signpost signal 39 in which thefield 109 indicates antenna select capability is to be disabled, the tag12 resumes using both of the antennas 43 and 44. Further, if the tag 12is using one antenna but detects that it is no longer within a magneticfield generated by any signpost, the tag 12 would resume using bothantennas 43 and 44.

As an alternative approach, the antenna select information at 33 couldidentify a specific one of the antennas 43 and 44 that is to bedisabled. The tag 12 would respond to receipt of a wireless signal 39with this antenna select information by disabling the specific antennaidentified in the field 109. The tag 12 would then continue operatingwith only one antenna, until it received a signpost signal 39 selectingthe other antenna, or a signpost signal indicating that both antennasshould be used. Further, if the tag 12 was using only one antenna butdetected that it was no longer within a magnetic field generated by anysignpost, the tag 12 would resume using both antennas 43 and 44.

The next field in the digital word 101 is a suppression on/off controlfield 110. The signpost 11 inserts into this field the suppressionon/off information stored at 32 in its memory 28. When the tag 12receives a signpost signal 39, it will normally proceed to transmit awireless tag signal 53 that contains the signpost identification code107 from that received signpost signal. But if the received signpostsignal contains a suppression on/off field 110 that indicatessuppression is enabled, the tag 12 will suppress transmission ofwireless tag signals 53, until it receives a further wireless signpost39 with a suppression on/off field 110 indicating that the tag 12 is todisable transmission suppression and resume transmission of tag signals.In addition, if the tag 12 is suppressing transmissions but detects thatit is no longer within a magnetic field generated by any signpost, thetag 12 would re-enable transmission of tag signals 54 (but might notactually transmit a tag signal 54 until it encounters another signpost,or until some other event occurs).

The next field in the digital word 101 is an error control field 111. Inthis regard, communications between the signpost 11 and other devicesare essentially one-way transmissions. Further, many applications forthe apparatus 10 of FIG. 1 involve environments that have relativelyhigh noise levels. Consequently, it is important for a receiving deviceto be able to evaluate whether the digital word 101 in a receivedsignpost signal is correct, or whether the word has errors. The errorcontrol field 111 is therefore provided in order to permit a degree offorward error correction (FEC). In the disclosed embodiment, the errorcontrol field 111 contains several parity bits, but it wouldalternatively be possible to use some other type of error controltechnique.

The last field in the word 101 is a packet end field 112. This fieldindicates to a receiving device (such as the tag 12) that thetransmission of the signpost signal 39 is ending. In the embodiment ofFIG. 1, the packet end field 112 contains several bits that are each abinary “0”.

FIG. 3 is a diagrammatic view of a digital word 119 that representsinformation transmitted by the tag 12 of FIG. 1 within its tag signal53. In particular, the bits of the digital word 119 are seriallymodulated onto the carrier signal. The bits of the word 119 aretransmitted serially from left to right in FIG. 3.

With reference to FIGS. 1 and 3, the word 119 begins with a field thatcontains a preamble 121. The preamble 121 is functionally comparable tothe preamble 106 in the word 101 of FIG. 2. The next field in the word119 is a tag type field 122. As mentioned earlier, the apparatus of FIG.1 can include a number of tags, and this group of tags can includevarious different types of tags. The tag type field 122 identifies theparticular type of tag that transmitted the wireless tag signal 53containing the word 119. The next field in the word 119 is an asset typefield 123, and indicates the type of asset to which the tag 12 iscurrently attached. For example, the field 123 would contain one code ifthe tag was attached to one type of vehicle, would contain a differentcode if the tag was attached to a different type of vehicle, wouldcontain yet another code if the tag was attached to a particular type ofshipping container, and so forth.

The next field 125 in the word 119 contains time information from theRTC 57 of the tag 12, identifying the particular point in time at whichan event occurred. As one example, and as discussed above, the receiver49 of the tag 12 is capable of detecting whether or not the tag 12 iscurrently within the magnetic field generated by a signpost 11. When thecontrol circuit 47 first detects that the tag 12 has entered themagnetic field of a signpost 11, that can be considered to be theoccurrence of an event, and the tag 12 can transmit one or more tagsignals 53 containing a word 119 in which the time information field 125indicates the precise time at which the event occurred. As a differentexample, when the sensor 58 of the tag 12 first detects some specificcondition, for example that an ambient temperature is outside aspecified range of acceptable temperatures, that could be treated as anevent causing the tag 12 to transmit one or more tag signals 53 in whichthe field 125 contains the time of the event. The next field 126 in theword 119 is an event identification field, and contains a codeidentifying the particular event that corresponds to the timeinformation present in the time information field 125.

In theory, when the tag 12 detects an event, it could promptly transmita tag signal 53 identifying the event in the field 126, but without anytime information field 125. The central system 14 could then associatethe event identification code 126 with the point in time at which thereader 13 received the tag signal 53. But as practical matter, asdiscussed above, it is often not possible to effect immediatetransmission of a tag signal 53 to the reader 13, for example due to thefact that the tag 12 must transmit tag signals 53 according to a timingprotocol such as the slotted aloha protocol. And even when the tag 12does transmit the signal 53, if the ambient environment is noisy (forexample because many tags are all transmitting), the tag 12 may have totransmit the tag signal 53 several times before that signal isaccurately received by the reader 13. Consequently, the reader 13 andthe central system 14 will learn of the occurrence of the event with avariable and unpredictable amount of time delay after the actualoccurrence of the event. But in the embodiment of FIG. 1, as discussedabove, the RTC 57 in the tag 12 is kept accurately synchronized with theRTC 71 in the central system 14, by sending timing information from thecentral system 14 through the network 19 to the signpost 11, and thenfrom the signpost 11 through wireless signals 39 to the tag 12.Therefore, the tag 12 can very accurately identify exactly when an eventoccurs, and can identify that point in time in the time informationfield 125. Thus, even though there can be a variable and unpredictableamount of delay before the reader 13 receives a tag signal 53 relatingto the event, the reader 13 and central system 14 will receive a highlyaccurate indication of the precise point in time at which thatparticular event occurred.

The next field in the digital word 119 is a signpost identification codefield 127. This field contains the signpost identification code 107(FIG. 2) from the wireless signpost signal 39 that was most recentlyreceived by the tag 12. The next field 128 in the word 119 containslocation information. This location information is an indication of thephysical location of the signpost that generated the wireless signpostsignal 39 most recently received by the tag 12. The location information128 will be discussed in more detail later. In some applications, it ispossible to optionally omit the signpost code 127 from the word 119,such that the reader 13 receives the location information in the field128, without any corresponding signpost identification code.

The next two fields 129 and 130 in the word 119 contain sequenceinformation. In the embodiment of FIG. 1, the field 127 contains themost recently received signpost code, the field 129 contains a differentsignpost code most recently received before the signpost code in field127, and the field 130 contains yet another signpost code most recentlyreceived before the signpost code in field 129. Thus, by examiningfields 127, 129 and 130 in a received word 119, the central system 14can identify the three signposts that were most recently encountered bythe tag 12, as well as the sequence in which those three signposts wereencountered, so as to ascertain the approximate path of travel of thetag 12, and the tag's direction of movement along that path of travel.

In an alternative configuration, the sequence information in the fields129 and 130 can be location information. For example, the field 128contains location information for the most recently encounteredsignpost, the field 129 would contain location information for adifferent signpost encountered most recently before the signpostassociated with field 128, and the field 130 would contain locationinformation for still another signpost encountered most recently beforethe signpost associated with field 129.

The next field in the word 119 is an error control field 131. In thedisclosed embodiment, this field contains a cyclic redundancy code (CRC)of a known type, which is calculated using the information in fields122-130. The error control field 131 gives the reader 13 a degree ofcapability to detect and correct some errors in a received word 119. Thelast field in the word 119 is a packet end field 132. This field signalsto the reader 13 that the transmission of signal 53 is ending. In thedisclosed embodiment, the packet end field 132 contains several binarybits that are each a binary “0”.

The invention is not limited to the particular word formats 101 and 119shown in FIGS. 2 and 3. The words 101 and 119 could each have otherfields in addition to the fields described above and shown in FIGS. 2and 3. Similarly, for certain applications, some of the fields shown inFIGS. 2 and 3 could optionally be omitted from one or both of the words101 and 119.

FIG. 4 is a diagrammatic top view showing a hypothetical application ofan apparatus of the type shown in FIG. 1, in order to help convey aclear understanding of certain aspects of the present invention. FIG. 4shows two rooms 151 and 152 from the same building, and these rooms arerespectively referred to as FLOOR 1 and FLOOR 2. Each of the rooms 151and 152 has two doors that are respectively identified as DOOR 1 andDOOR 2. It will be noted that DOOR 1 of each room is approximately twiceas wide as DOOR 2 thereof. The two rooms could be located in differentstories of the building, for example with FLOOR 2 located directly overFLOOR 1, and with DOOR 2 of each room being a respective entrance to acommon elevator. Alternatively, the two rooms could be on the same storyof the building, with a hallway extending from DOOR 2 of FLOOR 1 to DOOR2 of FLOOR 2.

FIG. 4 shows six signposts 161-166 that are distributed within FLOOR 1and FLOOR 2. Each of the signposts 161-166 is effectively equivalent tothe signpost shown at 11 in FIG. 1, except that the signposts 161-166each have a unique identification code 31. In FIG. 4, the number showninside each signpost 161-166 represents its respective identificationcode 31. In other words, the respective identification codes for the sixsignposts 161-166 are “714”, “558”, “672”, “788”, “948” and “536”. Thesignpost 161 is stationarily positioned near DOOR 2 of FLOOR 1, and thesignposts 162 and 163 are stationarily positioned on opposite sides ofDOOR 1 of FLOOR 1. Similarly, the signpost 164 is stationarilypositioned near DOOR 2 of FLOOR 2, and the signposts 165 and 166 arestationarily positioned on opposite sides of DOOR 1 of FLOOR 2.

Two additional signposts 167 and 168 are shown in broken lines near thesignpost 166. The signposts 167 and 168 are shown in broken linesbecause they are no longer present in FLOOR 2, but in the past they wereeach present at the location where signpost 166 is now installed. Inparticular, signpost 168 was originally present at this location, andwas then removed and replaced with the signpost 167. Later, the signpost167 was removed and replaced with the signpost 166.

As mentioned above, DOOR 1 is wider than DOOR 2 for each of FLOOR 1 andFLOOR 2. As also discussed above, the signposts 161-166 each transmit asignpost signal having an effective range of about 4 to 12 feet. If eachDOOR 1 is wider than about 10 to 12 feet, then a single signpostprovided on one side of that door would not be able to transmit asignpost signal far enough to reliably cover the entire width of thedoor opening. Consequently, DOOR 1 of FLOOR 1 has two signposts 162 and163 that are located on opposite sides thereof, and DOOR 1 of FLOOR 2also has two signposts 165 and 166 that are located on opposite sidesthereof. Each of these signposts can transmit a signpost signal farenough to cover at least half of the width of the adjacent door opening.Consequently, a tag passing through either of these doors willnecessarily receive a signpost signal from at least one of the twosignposts at that door.

FIG. 4 shows two readers 176 and 177 that are each equivalent to thereader shown at 13 in FIG. 1. The reader 176 is stationarily installedin approximately the center of FLOOR 1, for example on the ceiling.Similarly, the reader 177 is stationarily installed in approximately thecenter of FLOOR 2.

FIG. 4 also shows the tag 12 of FIG. 1. Typically, a number of tagswould be present within FLOOR 1 and FLOOR 2. However, for clarity inexplaining certain aspects of the invention, FIG. 4 shows only a singletag 12. The tag 12 can move with respect to the other componentsdepicted in FIG. 4. For example, the tag 12 may be mounted on a forkliftor other vehicle, or may be mounted on an object such as a shippingcontainer that is moved around. Tag 12 can move within FLOOR 1 andwithin FLOOR 2, and can also move from FLOOR 1 to FLOOR 2 and from FLOOR2 to FLOOR 1. The broken line 181 in FIG. 4 represent a hypotheticalpath of travel recently followed by the tag 12. This path of travel 181begins at a location 182 disposed approximately at the center of FLOOR2, then passes through DOOR 2 of FLOOR 2 and thus past tag 164, thenpasses through DOOR 2 of FLOOR 1 and thus past tag 161, and then extendsto a location near DOOR 1 of FLOOR 1, between the tags 162 and 163. Forthe sake of discussion, it is assumed that, as the tag 12 approachesDOOR 1 of FLOOR 1, it happens to receive a signpost signal from the tag163 before it receives a signpost signal from the tag 162.

As discussed above in association with FIG. 1, the tag 12 has a memory59 that stores four tables 61-64. Hypothetical examples of these tablesare shown in FIGS. 5-8, and in particular depict informationcorresponding to the hypothetical situation shown in FIG. 4. In moredetail, FIG. 5 shows an example of the location table 64. Each of thevisible rows in this table corresponds to a respective differentsignpost. The left field in each row contains the unique identificationcode of a particular signpost. Thus, from top to bottom, the eight rowsvisible in FIG. 5 respectively correspond to the signposts 167, 168,166, 162, 163, 161, 164 and 165 in FIG. 4. The middle and right fieldsof each row provide location information for the associated signpost. Inparticular, the middle field identifies whether the signpost is locatedon FLOOR 1 or FLOOR 2, and the right field identifies whether thesignpost is disposed by DOOR 1 or DOOR 2 of the floor identified in themiddle field. It will be noted that the hypothetical locationinformation given in FIG. 5 for each signpost corresponds directly tothe location of the corresponding signpost in the exemplary scenariodepicted in FIG. 4.

FIG. 6 is a diagrammatic view of the replacement/active table 61. Eachof the visible rows in table 61 corresponds to a respective one of thesignposts 161-168 in FIG. 4, where the left field in each row containsthe unique identification code for the signpost. The middle field ineach row is a binary bit that is set to a binary “1” if a signpost iscurrently active, or to a binary “0” if the signpost is currentlyinactive. Thus, for example, since the signposts 167 and 168 (havingrespective identification codes of “364” and “471”) have each beenpreviously removed from the scenario shown in FIG. 4, they are eachidentified in FIG. 6 as currently being inactive. In contrast, the othersix signposts of FIG. 4 are indicated to be active in FIG. 6. Althoughthe active/inactive information for the signposts is located in themiddle column of table 61 in FIG. 6, it would alternatively be possiblefor table 64 of FIG. 5 to have an additional column that contains thisactive/inactive information.

The right field in each row of table 61 contains an identification of areplaced signpost (if any). More specifically, if a given signpostreplaced another signpost, then the right field of the row for thereplacement signpost contains the identification code of the replacedsignpost. Thus, for example, the first visible row in table 61corresponds to signpost 167 (which has identification code “364”), andthe right field contains identification code “471” in order to indicatethat signpost 167 replaced signpost 168 (which has identification code“471”). In a similar manner, the third visible row of table 61 indicatesthat signpost 166 (having identification code “536”) replaced signpost167 (having identification code “364”).

FIG. 7 is a diagrammatic view of the equivalent table 62. Each row ofthis table identifies two or more signposts that, in the hypotheticalsituation shown in FIG. 4, are effectively equivalent. For example, thetwo signposts 162 and 163 in FIG. 4 are provided are opposite sides ofDOOR 1 of FLOOR 1, and are functionally equivalent. These signpoststransmit wireless signals with respective identification codes “558” and“672”. Either of these identification codes will tell the tag 12 thesame thing, or in other words that the tag is in the vicinity of DOOR 1of FLOOR 1. Consequently, the upper row visible in FIG. 7 contains theidentification codes “558” and “672” for these two signposts, in orderto indicate that these signposts are effectively equivalent. The lowerrow visible shown in FIG. 7 corresponds to DOOR 1 of FLOOR 2, wheresignpost 165 is effectively equivalent to signpost 166, as well as thereplaced signposts 167 and 168. That row of table 62 therefore includesthe identification codes for all four of the signposts 165-168.

In table 62 in FIG. 7, the rightmost field in every row is a one-bitflag. The tag 12 sets the flag bit in a given row the first time thatthe tag receives a signpost signal from any of the signposts identifiedin that row, and the tag thereafter ignores signpost signals from any ofthe signposts identified in that particular row. For example, the tag 12in FIG. 4 is approaching DOOR 1 of FLOOR 1, and receives a signpostsignal from the signpost 163 before it receives a signpost signal fromthe signpost 162. In other words, the first signpost signal receivedfrom either of the signposts 162 and 163 is from the signpost 163, andcontains identification code “672”. The tag 12 locates the row in table62 that contains identification code “672”, and sets the flag bit in theright field of this row, as shown in FIG. 7. The tag 12 is likely tosubsequently receive signpost signals from each of the signposts 162 and163 as the tag 12 passes through DOOR 1 of FLOOR 1. But each time itreceives such a signpost signal, it will find that the signpost code(“672” or “558”) is in a row of table 62 where the flag bit is set. Thetag 12 will therefore ignore each of these additional signpost signals,because they are all effectively redundant to the initial signpostsignal that caused the tag to set the flag bit.

As discussed above, the tag 12 can detect whether it is currently withinthe magnetic field produced by any signpost. As soon as the tag 12detects that it is no longer within the magnetic field of any signpost,it will reset all flag bits that have been set within table 62. Thus, inthe hypothetical scenario of FIG. 4, after the tag 12 has passed throughDOOR 1 of FLOOR 1 and has moved out of the transmission ranges of thesignposts 162 and 163, the tag will not be detecting the magnetic fieldof any signpost, and will reset any and all flag bits that had been setin table 62.

FIG. 8 is a diagrammatic view of the sequence table 63. The sequencetable 63 has three rows, and each row has one field that contains asingle signpost identification code. Each time the tag 12 receives asignpost signal with an identification code that is different from theidentification code in the top row of table 63, and that is notidentified as an equivalent in table 62 (FIG. 7), the tag 12 discardsthe identification code in the bottom row of table 63, moves each of theother two identification codes down one row, and inserts thenewly-received identification code in the top row. Consequently, thetable 63 always identifies, in sequence, the three signposts mostrecently encountered by the tag 12 (excluding any equivalent signpoststhat the tag encountered). With reference to the exemplary path ofmovement indicated at 181 in FIG. 4, the tag 12 passed the signpost 164(having identification code “788”), then passed the signpost 161 (havingidentification code “714”), then encountered the signpost 163 (havingidentification code “672”). Consequently, the sequence table 63 in FIG.8 shows the identification codes of these three signposts, arranged inthe sequence in which the tag 12 encountered those signposts.

In the embodiment of FIG. 1, the sequence table 63 of FIG. 8 containsthe identification codes of signposts that the tag 12 has passed.Alternatively, however, the sequence table 63 could contain locationinformation from the table 64 (FIG. 5) for each signpost that the tag 12has passed, such as the location information shown in the two rightfields of each row in table 64 of FIG. 5.

FIG. 9 is a flowchart showing how the tag 12 utilizes the four tablesshown in FIGS. 5-8 when the tag receives a signpost signal. Inparticular, in response to receipt of a signpost signal, the tag 12begins at block 201, and proceeds to block 202. In block 202, the taguses the identification code from the received signpost signal to locatethe row in table 61 (FIG. 6) that corresponds to the associatedsignpost, and then the tag checks that row to see if that signpost iscurrently active. If the signpost is not currently active, then itssignpost signal is irrelevant and should be ignored, and the tag 12exits the flowchart of FIG. 9 at block 203.

In contrast, if the tag determines at block 202 that the signpost inquestion is active, then control proceeds from block 202 to block 206,where the tag 12 checks table 61 to see if the signpost it hasidentified is a signpost that replaced some other signpost. If so, thencontrol proceeds to block 207, where the tag 12 retrieves from table 61the identification code of the replaced signpost, and shifts its focusto that replaced signpost. In particular, control returns to block 206,where the tag checks to see if the replaced signpost was itself used toreplace yet another signpost. When a determination is made at block 206that the tag 12 has identified a signpost that did not replace anothersignpost, control proceeds from block 206 to block 208.

As a practical example, and with reference to FIG. 4, assume that thetag 12 receives a signpost signal from signpost 166 (havingidentification code “536”). Using table 61, the tag 12 will determinethat signpost 166 replaced signpost 167 (having identification code“364”). The tag will shift its focus to signpost 167, and will thendetermine that signpost 167 replaced signpost 168 (having identificationcode “471”). The tag 12 will then shift its focus to signpost 168, andwill find that signpost 168 did not replace any other signpost.Accordingly, the tag will proceed from block 206 to block 208, and willcarry out further processing in the flowchart of FIG. 9 usinginformation relating to the original signpost 168, and will effectivelyignore the two replacement signposts 166 or 167. Stated differently,even though the tag 12 actually received a signpost signal from thesignpost 166, the tag will end up treating the signpost signal as thoughit was received from the signpost 168 (which is no longer actuallypresent or active in the scenario of FIG. 4).

In block 208, the tag 12 checks to see whether the received signpostsignal is equivalent to some other signpost signal that the tag hasalready received. More specifically, the tag 12 locates the appropriateidentification code in equivalent table 62, and checks the right fieldof that row in order to see if the flag bit is set. If so, then thereceived signpost signal can be ignored, and control proceeds to block203, where the tag exits the flowchart of FIG. 9. It should be notedthat, if the received signpost signal came from a signpost that hasreplaced another signpost, then pursuant to blocks 206 and 207 in FIG.9, the tag 12 will check the table 62 using the identification code ofthe replaced signpost. Thus, in the hypothetical scenario of FIG. 4, ifthe received signal was from the signpost 166, the tag 12 would haveidentified the replaced original signpost 168, and would be checkingtable 62 for the identification code “471” of the replaced signpost 168.

If the tag 12 determines in block 208 that the flag in the right fieldof the appropriate row in table 62 has not been set, then controlproceeds from block 208 to 209, where the tag 12 sets that particularflag in the table 62. Control then proceeds from block 209 to block 211.

In block 211, the tag 12 searches table 64 (FIG. 5) for the relevantsignpost identification code, and then retrieves the locationinformation associated with that identification code. Control thenproceeds to block 212, where the tag 12 updates the sequence table 63shown in FIG. 8. More specifically, as discussed above, the tag 12discards the information in the bottom row, moves each of the other twoitems of information down one row, and then inserts new information inthe top row. If the tag 12 is maintaining sequence information in theform of signpost identification codes, then the new signpostidentification code is inserted in the top row. Alternatively, if thetag 12 is maintaining sequence information in the form of locationinformation, then the location information retrieved at block 211 isinserted in the top row of table 63.

From block 212, control proceeds to block 213, where the tag 12 exitsthe flowchart of FIG. 9, and continues other processing related to thereceived signpost signal. For example, as part of this additionalprocessing, the tag 12 will transmit a tag signal 53 (FIG. 1) thatcontains the information shown in the digital word 119 of FIG. 3. Thelocation information in field 128 will be the location informationretrieved at block 211 in FIG. 9, and the sequence information in fields129 and 130 will be the information from the lower two rows in thesequence table 63 of FIG. 8. The information from the top row of thesequence table 63 will inherently appear in either field 127 or field128, depending on whether the sequence table 63 contains signpostidentification codes or signpost location information.

FIG. 10 is a diagrammatic fragmentary view of a hypothetical scenariorepresenting another possible application for the apparatus of FIG. 1.FIG. 10 shows a railway that includes a plurality of standard railroadties, two of which are identified at 256 and 257. The railroad tiessupport standard rails, two of which are visible at 258 and 259. Theadjacent ends of the ties 258 and 259 are spaced a short distance fromeach other, and a weight-activated switch 262 is provided in the regionbelow the space between the rails 258 and 259. A short rail section 263is supported on the switch 262. FIG. 10 also shows part of a train thatis traveling along the railway, including two conventional railway cars267 and 268 that are releasably coupled to each other by a knowncoupling mechanism. Each time that a wheel of a railway car passes overthe short rail section 263, part of the weight of the railway car willbe applied to the short rail section 263, and thus in turn to the switch262, so as to actuate the switch 262.

A vertical post 271 is provided near the switch 262, and has its lowerend fixedly anchored in the ground. The signpost 11 and the reader 13 ofFIG. 1 are each mounted on the post 271. Each of the railway cars of thetrain has a tag 12 mounted thereon. As indicated diagrammatically bybroken lines in FIG. 10, the signpost 11 and the reader 13 are eachoperatively coupled to the central system 14 through the network 19, asdiscussed above in association with FIG. 1. In addition, an output ofthe switch 262 is operatively coupled to the central system 14 throughthe network 19. As discussed above in association with FIGS. 1-3, timinginformation from the RTC 71 in the central system 14 is supplied to thesignpost 11 through the network 19. When each tag 12 is close enough tothe signpost 11 to receive the wireless signpost signals 39 transmittedby the signpost, the tag receives accurate timing information in thefield 108 of the word 101 within the signpost signals.

As explained above, each time that a wheel on one of the railway carspasses over the rail section 263, the switch 262 is actuated. Since thecentral system 14 receives the output of the switch 262, The centralsystem 14 can maintain an accurate count of the number of times that theswitch 262 is actuated as the train passes by, and can determine fromthis the number of railcars that pass the switch 262. In addition, thetag 12 on each railcar will respond to the wireless signpost signals 39from the signpost 11 by transmitting a wireless tag signal 53. The tagsignal 53 contains information of the type discussed above inassociation with FIG. 3, including the unique identification code 124 ofthe tag 12, as well as the time information 125. When the reader 13receives the tag signal 53, the reader 13 will take this informationfrom the signal and forward it to the central system 14 through thenetwork 19. Since the tag 12 has received accurate time information fromthe central system 14 through the signpost 11 and the wireless signals39, the time information 125 that the tag 12 includes in the wirelesssignal 53 will be a very accurate indication of the point in time atwhich a specified event occurred. For example, in the embodiment of FIG.10, this event occurs when the tag 12 first detects the magnetic fieldproduced by the signpost 11.

In the embodiment of FIG. 1, the signpost identification code stored at31 in the memory 28 of the signpost 11 is a predetermined code uniquelyidentifying that particular signpost. It may be preset when the signpostis manufactured. In an alternative embodiment, the signpostidentification code stored at 31 is a programmable value that can beselectively set by the central system 14 through the network 19.Further, in a related variation of FIG. 4, equivalent signposts, such asthe signposts 162 and 163, do not have different signpost codes.Instead, the central system 14 sets the signpost codes 31 within thesetwo signposts to be identical. Similarly, the central system 14 sets thesignpost codes 31 within the two equivalent signposts 165 and 166 to beidentical, but different from the code in signposts 162-163.

When the tag 12 is near DOOR 1 of FLOOR 1, it will receive the samesignpost code in any signpost signal, regardless of whether that signalcomes from the signpost 162 or the signpost 163. Similarly, when the tag12 is near DOOR 1 of FLOOR 2, it will receive the same signpost code inany signpost signal, regardless of whether that signal comes from thesignpost 165 or the signpost 166. Once the tag 12 receives one signpostsignal containing a given signpost code, it will ignore all othersignpost signals it subsequently receives that contain the same code,until it receives a signpost signal with a different code. Consequently,in this modified embodiment, the tag 12 would not need to maintain theequivalent table 62 shown in FIG. 7.

FIG. 11 is a diagrammatic view of a scenario that represents anapplication for an alternative embodiment of the system shown in FIG. 1.In FIG. 11, a standard forklift 301 is carrying a container 302. Avertical post 306 is stationarily installed adjacent a path of travel ofthe forklift 301, and a signpost 311 is mounted on the post 306. Thesignpost 311 is generally similar to the signpost 11 discussed above inassociation with FIG. 1, except that wireless signpost signals 312transmitted by the signpost 311 do not include a signpost identificationcode 107 (FIG. 2). A tag 316 is mounted on the forklift 301. The tag 316is generally similar to the tag 12 discussed above in association withFIG. 1, except that wireless tag signals 317 transmitted by the tag 316do not include a signpost identification code 127 (FIG. 3).

A reader 321 is mounted on the post 306. The reader 321 is generallysimilar to the reader 13 discussed above in association with FIG. 1,except that the reader 321 is configured to have a relatively shortreception range for wireless tag signals 317. For example, an antennaand/or receiver circuit of the reader 321 may have a modified structurethat serves to reduce the sensitivity of the reader 321 to wireless tagsignals 317. In addition, or as an alternative, the tag 316 could havean antenna or transmitter circuit with a modified structure that reducesthe effective transmission range of the wireless tag signals 317. Thetag 316 and/or reader 321 are thus configured so that, in order for thereader 321 to reliably receive tag signals 317, the reader 321 must bewithin approximately 4 to 12 feet of the tag 316 that is transmittingthe signals.

As discussed earlier, in order for the tag 316 to reliably receive thesignpost signals 312 transmitted by the signpost 311, the tag 316 mustbe within approximately 4 to 12 feet of the signpost 311. Thus, theeffective transmission range of the tag signals 317 is approximately thesame as the effective transmission range of the signpost signals 312. Inthe arrangement of FIG. 11, when the tag 316 first receives a signpostsignal 312 from the signpost 311, the tag 316 responds by transmittingat least one tag signal 317. When the reader 321 receives that tagsignal 317, it forwards the information from the tag signal to thecentral system 14. The central system knows that the tag transmitted thereceived tag signal when the tag received a signpost signal 312 from thesignpost 311, and that receipt of the signpost signal is only possibleif the tag (and the associated railway car) are within 4 to 12 feet ofthe signpost 311. The central system also knows that, in order for thereader 321 to have received the tag signal, the tag that transmitted thetag signal must be within about 4 to 12 feet of the reader 321.Therefore, since the central system 14 knows the location of thesignpost 311 and the reader 321, and also knows that the tag 316 isnecessarily within 4 to 12 feet of each of the signpost 311 and reader321, the central system 14 knows the location of the tag and thus thelocation of the railway car on which the tag is mounted, even though thetag signal 317 does not include a signpost code 127 that identifies thesignpost 311.

Although selected embodiments have been illustrated and described indetail, it should be understood that a variety of substitutions andalterations are possible without departing from the spirit and scope ofthe present invention, as defined by the following claims.

1. An apparatus comprising a tag having circuitry that includes: areceiver section configured to receive wireless signpost signals thatinclude timing data; a further section that responds to an event otherthan receipt of a wireless signpost signal by determining timeinformation associated with the event as a function of the timing datafrom a previously-received wireless signpost signal; and a transmittersection operable to transmit wireless tag signals that each include atag code associated with said tag, said transmitter section beingresponsive to the determination of time information by said furthersection for including the time information in at least one said tagsignal.
 2. An apparatus according to claim 1, wherein said wirelesssignpost signals are near field signals of primarily magnetic character,said receiver section being configured to receive said signpost signalsof magnetic character.
 3. An apparatus according to claim 1, whereinsaid circuitry of said tag includes a real time clock circuit; whereinsaid tag uses said timing data from received signpost signals to updatesaid real time clock circuit; and wherein said further section obtainssaid time information from said real time clock circuit.
 4. An apparatusaccording to claim 1, wherein said transmitter section configures eachsaid tag signal containing said time information to also include anidentification of the event associated therewith.
 5. An apparatusaccording to claim 1, wherein said circuitry includes a sensor thatdetects said event, said further section determining said timeinformation in response to detection of said event by said sensor.
 6. Anapparatus according to claim 1, wherein said wireless signpost signalseach include a signpost code.
 7. An apparatus according to claim 6,wherein said event occurs when said tag first receives a signpost signalcontaining the timing data and a selected signpost code.
 8. An apparatusaccording to claim 1, further including a signpost configured totransmit wireless signpost signals that include said timing data; and areader configured to receive the wireless tag signals transmitted bysaid tag.
 9. A method comprising: receiving in a receiver section of atag wireless signpost signals that include timing data; responding tothe occurrence of an event other than receipt of a wireless signpostsignal by determining in a further section of said tag time informationassociated with the event as a function of the timing data from apreviously-received wireless signpost signal; and transmitting from atransmitter section of said tag wireless tag signals that each include atag code associated with said tag, including responding to saiddetermining of said time information by said further section by causingsaid transmitter section to include the time information in at least onesaid tag signal.
 10. A method according to claim 9, including:configuring said wireless signpost signals to be near field signals ofprimarily magnetic character; and configuring said receiver section toreceive said signpost signals of magnetic character.
 11. A methodaccording to claim 9, including: configuring said tag to have a realtime clock circuit; using said timing data from received signpostsignals to update said real time clock circuit; and causing said furthersection to obtain said time information from said real time clockcircuit.
 12. A method according to claim 9, wherein said responding tosaid determining of said time information by said further sectionincludes causing said transmitter section to configure each said tagsignal containing said time information to also include anidentification of the event associated therewith.
 13. A method accordingto claim 9, including recognizing as the occurrence of said event thedetection of a condition by a sensor in said tag.
 14. A method accordingto claim 9, including configuring said wireless signpost signals to eachinclude a signpost code.
 15. A method according to claim 14, includingrecognizing as the occurrence of said event a situation where said tagfirst receives a signpost signal containing the timing data and aselected signpost code.
 16. An apparatus comprising: a signpost thattransmits wireless signpost signals having a selected transmissionrange, wherein said signpost comprises a real time clock that issynchronized to a real time clock in a network; a reader provided inproximity to said signpost and having a receiver operable to receivewireless tag signals with a reception range that is approximately thesame as said selected transmission range; and a tag that is movablerelative to said signpost and said reader, that has a receiver sectionconfigured to receive wireless signpost signals, and that has atransmitter section responsive to receipt by said receiver section of awireless signpost signal for transmitting a wireless tag signal thatincludes a tag code associated with said tag.
 17. An apparatus accordingto claim 16, wherein said wireless signpost signals are near fieldsignals of primarily magnetic character, said receiver section of saidtag being configured to receive said signpost signals of magneticcharacter.
 18. An apparatus according to claim 16, wherein said tagsignals are radio frequency signals.
 19. An apparatus according to claim16, wherein said wireless signpost signals transmit a digital word tothe tag that contains time information.
 20. A method comprising:transmitting from a signpost wireless signpost signals having a selectedtransmission range, wherein said signpost comprises a real time clock;synchronizing said real time clock at a predetermined event to a realtime clock in a network; receiving in a reader located proximate saidsignpost wireless tag signals with a reception range that isapproximately the same as said selected transmission range; moving a tagrelative to said signpost and said reader; receiving in a receiversection of said tag the wireless signpost signals; and responding toreceipt by said receiver section of a wireless signpost signal bytransmitting from a transmitter section of said tag a wireless tagsignal that includes a tag code associated with said tag.
 21. A methodaccording to claim 20, including: configuring said wireless signpostsignals to be near field signals of primarily magnetic character; andconfiguring said receiver section of said tag to receive said signpostsignals of magnetic character.
 22. A method according to claim 20,including configuring said tag signals to be radio frequency signals.23. A method according to claim 20, including configuring said wirelesssignpost signals to transmit a digital word to the tag that containstime information.